For the last several days I got stuck in an annoying situation that I have to re-enter my account info (username and password) for every single website I use (about 10 I guess). For some reasons it seems that my newly-upgraded Firefox 3.5 Preview would not save any cookies at all.
So I did a little research the reason turned out to be some corrupted file called cookies.sqlite. All you have to do is to seek and destroy it. That’s all! (And by saying “destroy” I didn’t mean that you have to actually chop its head down or something, just delete it — even to the recycle bin is OK.) The file can often be found at your %appdata%\Mozilla\Firefox\Profiles\ and in a folder named whatever.default.
Hope this is helpful if you have the same bug with Firefox.